1. Field of the Invention
This invention relates to a radiation image storage panel comprising a support, a phosphor layer which comprises a binder and a stimulable phosphor dispersed therein and a protective film, superposed in this order.
2. Description of Prior Arts
For obtaining a radiation image, there has been conventionally employed a radiography utilizing a combination of a radiographic film having an emulsion layer containing a photosensitive silver salt and a radiographic intensifying screen.
As a method replacing the conventional radiography, a radiation image recording and reproducing method utilizing a stimulable phosphor as described, for instance, in U.S. Pat. No. 4,239,968, has been recently paid much attention. In the radiation image recording and reproducing method, a radiation image storage panel comprising a stimulable phosphor (i.e., stimulable phosphor sheet) is used, and the method involves steps of causing the stimulable phosphor of the panel to absorb radiation energy having passed through an object or having radiated from an object; sequentially exciting the stimulable phosphor with an electromagnetic wave such as visible light or infrared rays (hereinafter referred to as "stimulating rays") to release the radiation energy stored in the phosphor as light emission (stimulated emission); photoelectrically detecting the emitted light to obtain electric signals; and reproducing the radiation image of the object as a visible image from the electric signals.
In the radiation image recording and reproducing method, a radiation image is obtainable with a sufficient amount of information by applying a radiation to the object at considerably smaller dose, as compared with the conventional radiography. Accordingly, this method is of great value especially when the method is used for medical diagnosis.
The radiation image storage panel employed in the radiation image recording and reproducing method has a basic structure comprising a support and a phosphor layer provided on one surface of the support. Further, a transparent film is generally provided on the free surface (surface not facing the support) of the phosphor layer to keep the phosphor layer from chemical deterioration or physical shock.
The phosphor layer comprises a binder and stimulable phosphor particles dispersed therein. The stimulable phosphor emits light (gives stimulated emission) when excited with an electromagnetic wave (stimulating rays) such as visible light or infrared rays after having been exposed to a radiation such as X-rays. Accordingly, the radiation having passed through an object or having radiated from an object is absorbed by the phosphor layer of the panel in proportion to the applied radiation dose, and a radiation image of the object is produced in the panel in the form of a radiation energy-stored image. The radiation energy-stored image can be released as stimulated emission by sequentially irradiating (scanning) the panel with stimulating rays. The stimulated emission is then photoelectrically detected to give electric signals, so as to reproduce a visible image from the electric signals.
The radiation image storage panel is generally employed repeatedly in the radiation image recording and reproducing method, because the panel is hardly deteriorated by exposure to a radiation such as X-rays and stimulating rays. Accordingly, the protective film of the panel provided on the surface of the phosphor layer preferably has a large thickness from the viewpoint of physical or chemical protection of the phosphor layer, such protection being a principal purpose of the protective film. However, the radiation image storage panel is generally read out from the protective film-side surface of the panel upon exposure to stimulating rays, and hence from the viewpoint of image quality such as sharpness, the thickness of the protective film is desired to be as small as possible.
As the transparent protective film of the panel having a small thickness and high hardness, there has been conventionally employed a polyethylene terephthalate film having a thickness of approx. 10-20 .mu.m.
However, it has been now discovered that this useful polyethylene terephthalate film has a drawback in that it shows a high water vapor transmission. Especially when the stimulable phosphor to be contained in the phosphor layer is easily deteriorated upon absorption of water to cause lowering of emission luminance thereof (i.e., in the case that the phosphor lacks water-vapor resistance), it is necessary to provide a protective film having a low water vapor transmission onto the surface of the phosphor layer. In more detail, provision of the protective film having a low water vapor transmission on the phosphor layer can prevent water (i.e., moisture) in an atmosphere from permeating the phosphor layer so as to protect the phosphor layer from deterioration caused by water.